THEME A - CONTROL OF PATTERN (Arthur Lander, Theme Leader) I Understanding pattern - the regular arrangement of cells and cellular behaviors in space - is one of the oldest problems in biology. Over the past half-century, two views of pattern formation have flourished [63,64], both appealing to the actions of molecules, termed morphogens, that spread within tissues. In boundary-organized' patterning, long-range morphogen gradients provide positional Information, giving cells their coordinates with respect to a frame of reference. In self-organizing patterning, morphogens act over shorter range to induce or repress expression of themselves or other morphogens, which in turn influences their own activity. Such feedback creates instabilities that lead to spontaneous division of cell groups into repeating units. Aim A l , below, explores changing concepts in how long-range morphogens control pattern. Aim A2 pursues the idea that patterning is often neither boundary- nor self-organized, but a combination of both. A third aim explores how pattern control and growth control are coordinated (it is designated Aim ABS, to reflect the fact that it will be a joint effort between Theme A [pattern] and Theme B [growth control]).